Various applications, particularly those used for wafer etching and cleaning in the semiconductor industry, require high levels of liquid chemical purity during production processing. These chemicals are often volatile, corrosive, and unstable. To reduce contaminating particulates, many generated during the production process itself as well as in the initial chemical charge, various in-process recirculating filtering arrangements have evolved in the prior art. These units are primarily constructed of plastic and use heaters immersed directly in the working chemistry.
However, as the semiconductor process industry turned to more aggressive chemistries, used at higher temperatures and requiring higher purity levels, there evolved a need for new containment structures to withstand these conditions and deliver the requisite purity levels. This need was filled by the development of quartz chambers as the basic containment vessel, with through-the-wall heating to achieve more even temperature distribution and reduce heater failure through exposure to the chemical solutions. Along with this development came the belief in the process industry that the chemical solution should enter and flow upward from the bottom of the containment vessel, through the array of wafers being processed, and out over a weir at the top of the vessel, so that process-generated particles could be filtered out before the solution re-entered the vessel.
Prior art quartz recirculating baths are divided into two general approaches. One version consists of a quartz working vessel supported inside a larger quartz outer vessel. Heaters are secured to the outside of the outer vessel wall to heat the liquid contained between the walls of the two vessels. The top edge of the outer vessel terminates in an outward-facing quartz flange which is sealed to a plastic outer case. The outer case is packed with thermal insulation, and also serves to protect the heaters from the chemical solutions. The inner vessel contains the working load of wafers. The filtered chemistry is introduced through fittings passing through the bottom of the outer vessel in sealed fashion to an opening in the bottom of the inner vessel. The flow path passes through the wafer array, over the four sides of the top lip of the inner vessel and down the outer surface of the inner vessel to form a reservoir pool of liquid between the inner and outer vessel walls.
The liquid in the reservoir absorbs heat conducted from the heaters through the outer wall. A fitting at the bottom of the outer tank draws off the liquid to a pump/filter fluid circuit. While this system provides highly efficient flow, it suffers from having the heat put into the solution just before entering the pump/filter, and then returning the filtrate to the tank at its lowest temperature. The pump, being the most heat-limited component of the typical recirculating system, receives the recirculating liquid at its hottest, and the wafers receive the liquid at its coolest. This arrangement shortens the mean time between repairs, and is also wasteful of power.
In addition, the performance of this system is very sensitive to the amount of liquid in the system. The inter-wall reservoir has a tall, narrow effective cross-section, and any liquid removed from the system by evaporation, "drag out" on the wafers, or leakage, results immediately in a significant reduction of the liquid level between the walls of the two quartz containers. Variations in liquid level can have a profound effect on the heat transfer into the liquid through the outer wall. More importantly, the liquid level may drop below the height of the heater placement, causing local heat build-up in the heater leading to early bath failure and possible fire conditions. The monitoring of the liquid level between the walls of this type of bath must be rigorous and provision must be made for make-up chemistry on a frequent basis.
The second common recirculating bath design employs a single working vessel, and a trough secured near the top of the outer surface of the vessel walls. The trough is exposed at the top to the local environment on three sides, and the bottom of the trough forms a seal with a plastic outer case and heating chamber within the outer case. Heaters are mounted within the outer case, on the outer surface of the working vessel, and insulation is packed into the outer case to retain heat. Liquid flows up from the bottom of the vessel, over its upper lip on four sides, and down into the shallow sloped trough where it is carried to a pick-up fitting, collected and plumbed back into the pump/filter circuit, and thence to the bottom of the working vessel, where it is heated. This arrangement has the advantage of heating the chemistry as it returns from the filter, so that the pump receives the liquid at its closest temperature. For low-flow systems, this system works quite well.
The second bath design encounters problems at high liquid flow rates, such as standing waves or turbulence in the liquid in the trough. Standing waves can deprive the pump of liquid, and turbulence can homogenize air into the returning liquid. Entrained air can seriously affect both pumping and filtering functions. Moreover, when a comparatively small amount of liquid is lost from the system, the liquid level falls in the trough and air can be drawn into the pickup fitting and thence into the pump. This problem is exacerbated by a vortexing phenomenon caused by high flow rates at the pickup fitting which can both block liquid flow to the pump and cause air ingestion, even though the trough liquid level would otherwise be sufficient to feed the pump intake.